It has been known, as disclosed in Melvin D. Ricks U.S. Pat. No. 3,842,504, issued Oct. 22, 1974, to provide a turbine-driven dental handpiece with a pliant chamber extending about the handle thereof, or along a floor surface adjacent to a dental chair, such chamber being completely sealed except for its communication with a conduit leading to pressure-responsive means for controlling the flow of drive air to the handpiece. A fluid, which would ordinarily be non-compressible and therefore a liquid rather than a gas, is contained within the chamber and conduit leading to the pressure-responsive control means. When the pliant chamber is squeezed, the liquid is displaced along the conduit and into the control means which in turn regulates the flow of drive air to the turbine of the handpiece. The extent to which the pliant chamber is compressed determines the extent of liquid displacement and, consequently, the operating speed of the handpiece.
While such a construction provides the advantages of direct fingertip or foot control of the handpiece speed, it has the disadvantages of being highly sensitive to leaks and to temperature and pressure changes. The effectiveness of such a system depends on the use of a sealed chamber from which the fluid is displaced as the chamber is reduced in size. Quite obviously, a loss of fluid would adversely affect the operation of the unit, just as the loss of fluid in any sealed hydraulic system would be expected to have adverse consequences. Since such a fluid would in most instances be a liquid, the escape of such fluid, by leakage or diffusion, might also result in the formation of an objectionable surface film on the instrument, cause staining and other damage to clothing and equipment, and produce other undesirable results in addition to loss of handpiece speed control.
In addition the need to prevent leakage from such a system requires the use of elements designed to insure positive sealing and such elements, when they are appended to the dental handpiece itself, are undesirable to the extent that they intend to increase weight in a handpiece which must be manipulated with great precision and sensitivity. Since such a system practically necessitates the use of a liquid (i.e., non-compressible) medium, the weight of such medium in a dental handpiece is a further disadvantage.
Other disadvantages may also be present. Thus, unintentional operation of the handpiece might occur simply by reason of thermal expansion of the fluid within the sealed system. Also, where the system utilizes fingertip control of handpiece speed, the provision of an additional liquid-filled conduit within the hose leading to the handpiece would be expected to add further bulk, weight, and stiffness, thereby making manipulation of the handpiece more difficult.
Other patents revealing the state of the art are U.S. Pat. Nos. 1,078,785, 2,591,119, 3,032,878, 3,125,809, 3,277,254, 2,204,644, 2,787,756, 2,937,444, 2,622,619, 3,188,011, 3,568,318, 3,250,005, 3,256,603, 3,346,958 and 3,676,931.